Technical field
This invention relates to a process for manufacturing an interconnection substrate to connect a chip onto a reception substrate.
Interconnection substrates act as intermediaries for the installation of one or several electronic chips on a support, for example such as printed circuit boards. Their essential function is to adapt the very tight pitch between chip inputs and outputs with the much more widely spaced connection terminals made on printed circuit boards.
Therefore the invention is particularly applicable to electronics in which VLSI chips are used on conventional printed circuits.
As described above, the essential function of interconnection substrates is to adapt the pitch of reception substrate connection terminals to the very small pitch of chip inputs and outputs. Another also very important function is to absorb mechanical stresses that occur between the chip and the reception substrate. These stresses are mainly due to differences in thermal-mechanical expansion between the chips and the printed circuit.
As shown in section in FIGS. 1 and 2, interconnection substrates 10 comprise a side 12 with input terminals 14 connected to one or several chips 16, and a side 18, opposite side 12, with output terminals 20 connected to a reception substrate 22. Chip 16 may be connected to the input terminals 14 either through wire connections 24, called "wire bonding" (FIG. 1), or through meltable microballs 26 using a technique known as "Flip-Chip" (FIG. 2). Chip 16 is advantageously protected by a cover 17.
Interconnection substrates 10 are mounted on the reception substrate 22 by balls 28 made of a meltable material which connect output terminals 20 to reception substrate conducting tracks not shown on the figures.
The meltable balls 28 connecting the interconnection substrate to the reception substrate are usually larger than the microballs 26 which connect the chips to the interconnection substrate. Therefore, to avoid any confusion, meltable balls 28 are called "macroballs" in the remainder of this description. The typical diameter is of the order of 200 to 800 .mu.m.
These macroballs are beneficially used instead of any other type of link such as links by pins. They are better capable of absorbing differential expansion stresses between the chips and the reception substrate and therefore reducing stresses exerted on the chips.
The connection between interconnection substrates on support substrates by means of macroballs is known as BGA ("Ball Grid Array"). Reference can be made to document (1) cited at the end of this description.
Document (1) also describes the various types of interconnection substrates and microballs used for a BGA interconnection.
In general, the interconnection substrate is based on bimaleid-triazine (BT) or epoxy glass. However, a distinction is made between BGA on a rigid interconnection substrate and BGA on a flexible circuit.
Rigid interconnection substrates have a multi-layer structure formed for example by silk screen printing on a rigid epoxy and glass fiber board.
In this type of structure, input terminals are connected to output terminals through plated through holes and possibly intermediate metal levels made in the interconnection substrate. By plated through holes is meant usually mechanically made through holes that are plated to enable electrical contact between the front and back sides or between two metal levels.
On FIGS. 1 and 2, plated through holes and intermediate metal levels are identified as reference 30. Since these figures are cross-section views, and not all terminals are necessarily in the plane of the section, some plated through holes only appear partially.
Known interconnection substrates mostly use known manufacturing techniques such as silk screen printing and electrolytic growth.
Rigid substrates formed from a multi-layer ceramic structure obtained by hot pressing are also known. Such structure is shown for example in page 12 in document (1).
Pages 65 and 66 in document (1) describe a BGA technique on a flexible circuit. The flexible circuit that forms the interconnection substrate has a copper-polyimide-copper type structure similar to that used in TAB (Tape Automatic Bonding) techniques, using a layer of a material marketed under the name of Kapton as insulator, and copper conducting films silk screen printed on one or both sides of the flexible circuit, the connection between the two sides then being made by plated through holes.
After assembling the chips on a flexible circuit strip and packaging these chips, the strip is cut out provided with macroballs and then placed on a reception printed circuit.
Finally, regardless of the intermediate substrate structure used, it is manufactured using silk screen printing, rolling and through hole plating techniques. The use of plated through holes is the only known way of transferring signals processed by chips from input terminals to output terminals.
Known interconnection substrates have poor performances for the transmission of high frequency signals from chips to the reception substrate, or between chips.
It may be noted that chips (or integrated circuits) use metal tracks usually made of aluminium with a thickness of the order of 1 .mu.m. These tracks form transmission lines, and are isolated for example using a mineral insulation of the same thickness. The electrical transmission properties of these lines enable the transport of high frequency signals over a maximum length of 15 mm.
Therefore, integrated circuits are designed so that they are hardly larger than 15 mm along the side. However, interconnection substrates which must make connections between a large number of chips create frequency limitation problems. Therefore, in order to improve their electrical performances, the resistivity of their conducting tracks has to be reduced. Reducing the resistivity means increasing the thickness of metal tracks, and the thickness of the dielectric materials separating the conducting levels. This can be done using normal techniques for printed circuits, co-sintered multi-layer ceramics or flexible circuits, by increasing the thickness of the polyimide insulating layers (10 .mu.m). However, this type of action limits the pitch resolution between the terminals, and the chip input and output density, which is contradictory to the primary function of interconnection substrates.
Thus, an additional intermediate interconnection substrate is used for interconnection substrates designed to hold a large number of chips, commonly called MCM (Multi-Chip-Module) interconnection substrates.
FIG. 3 shows an example of an MCM type structure. This structure, in the same way as the structures shown in FIGS. 1 and 2, comprises a reception substrate 22 provided with macroballs 28. However, several chips 16, 16' must be connected not only to the reception substrate 22, but also to each other and are connected to an additional intermediate interconnection substrate 31, rather then being directly connected to the interconnection substrate 10.
The intermediate interconnection substrate 31 is made on a silicon board and has three to five copper conducting levels separated by thick polyimide insulating layers. The purpose of the intermediate substrate is to enable fast (i.e. high frequency) signal exchanges between chips and from chips to the interconnection substrate 10.
An intermediate interconnection substrate 31, although it does provide a solution to the fast interconnection of a large number of chips, forms an additional expensive element in transferring chips onto the reception substrate. Also, three interconnection operations are necessary: an interconnection of the chips onto the additional intermediate substrate, an interconnection of the intermediate substrate onto the interconnection substrate, and then a transfer of the interconnection substrate onto the reception substrate. This increases the time and cost of the operation.
Thin film MCM type structures are also known. Examples of these structures are described for example in document (2) referenced at the end of this description.
One purpose of this invention is to provide a process for making an interconnection substrate that does not have the limitations described above.
Another purpose of this invention is to provide a process for making an interconnection substrate enabling interconnection of a plurality of chips on a reception substrate, and capable of transmitting high frequency signals. By high frequency signals is meant signals with a frequency exceeding 50 MHz.